1. Field of the Invention
This invention relates to solid electrolytes and more particularly, to a polymer solid electrolyte composition which comprises aluminum-based conductive carriers and can thus exhibit high ionic conductivity with good film-forming properties, mechanical strength and flexibility.
2. Description of the Prior Art
Use of solid electrolytes to constitute totally solid cells contributes to improving the reliability of the cell without leakage of the content in the cell. Since the cell can be made thin and a plurality of cells may be built up, attention has been directed to solid electrolytes for use as a material in the fields of cells and other electrochemical devices.
The characteristic properties required as a solid electrolyte generally include (a) high ionic conductivity without involving electron conductivity, (b) good film-forming properties by which a thin film can be formed, and (c) good flexibility.
Broadly, solid electrolytes can be divided into two groups including an inorganic group and an organic group. The inorganic solid electrolytes have relatively high ionic conductivity but are poor in mechanical strength because they are crystalline in nature. This makes it difficult to obtain flexible films. This is very disadvantageous when inorganic solid electrolytes are applied to devices.
In contrast, polymer solid electrolytes made of organic materials are able to form flexible thin films. The thus formed thin film is imparted with good mechanical properties owing to the flexibility inherent to polymer. The thin film consisting of the polymer solid electrolyte can be appropriately adapted for the volumetric variation caused during the course of the ion-electron exchange reaction between the electrode and the polymer solid electrolyte. For this reason, the polymer solid electrolytes have been expected as a promising solid electrolyte material for high energy density cells, particularly, thin cells.
Composite materials comprising polyethylene oxide (--CH.sub.2 CH.sub.2 O--).sub.n, hereinafter abbreviated as PEO) and alkali metal salts such as Li salts, Na salts and the like are known as a polymer solid electrolyte which exhibits high alkali metal ionic conductivity. Various types of polymer solid electrolytes including the above composite material have been theoretically studied with respect to the mechanism of ionic conductivity and the molecular structure. Extensive studies have also been made on the application of the polymer solid electrolytes to electrochemical devices such as cells. The ionic conduction of polymer solid electrolytes is now considered to occur in the following manner: the alkali metal salt in a polymer matrix selectively ionizes an amorphous sites in the polymer matrix and moves by diffusion along the electric field in the matrix thereby achieving the ionic conduction. For instance, it has now been accepted that with composite materials made of PEO and alkali metal salts, the alkali metal ions form a complex with the oxygen atom at the ether bond of the main chain which has a high dielectric constant and the molecule chain suffers segment movement by means of heat at the amorphous sites, thereby showing the ionic conductivity.
However, the polymer solid electrolytes have the problem that they are smaller in ionic conductivity in the vicinity of room temperature than solid electrolytes made of inorganic materials. In addition, improvements in the ionic conductivity bring about the problem that film-forming properties and flexibility are lowered instead.
For instance, with the composite material film consisting of PEO and alkali metal salts wherein the composite material has a molecular weight of about 10000, good film-forming properties are obtained with an ionic conductivity being as high as 10.sup.-3 to 10.sup.-4 Scm.sup.-1 at temperatures of 100.degree. C. or higher. Since the composite material is crystalline in nature, however, the ionic conductivity abruptly lowers at temperatures not higher than 60.degree. C. and is decreased to a very small value of not higher than approximately 10.sup.-7 Scm.sup.-1 at room temperature. This disenables the composite material to be used as a material for ordinary cells which are employed at room temperature. In order to improve the ionic conductivity, attempts have been made to suppress the crystallinity by introduction of crosslinking agents such as toluene-2,4-diisocyanate(TDI) but in vain. Although the ionic conductivity in the vicinity of room temperature can be improved by forming the composite material film of a composite material whose molecular weight is not higher than approximately 10000, film-forming properties are considerably lowered, making it difficult to form a film. Moreover, for improving the ionic conductivity, the concentration of an alkali metal salt may be increased. However, this will cause the glass transition point, Tg, of the polymer to increase, thus resulting in a lowering of the ionic conductivity. As will be apparent from the foregoing, it is not possible to increase both the carrier density and the ionic conductivity.
Other types of polymer solid electrolytes are known, which are similar to the composite materials consisting of PEO and alkali metal salts. Such electrolytes are ones which have PEO structures at side chains. This polymer solid electrolyte has an ionic conductivity ranging from 10.sup.-5 to 10.sup.-4 Scm.sup.-1 and is thus slightly improved over the composite material consisting of PEO and alkali metal salts. However, this ionic conductivity is not satisfactory in practical applications. In addition, film-forming properties and flexibility are not satisfactory.
On the other hand, attention has been paid to lithium ion cells or nickel hydrogen cells for use as a high capacitance cell. Now, there is a strong demand for developments of materials for secondary cells which are small in size, light in weight and high in capacitance. Aluminium cells have been recently studied as one of newly developing cells. Theoretically, aluminium cells are assumed to have a high density capacitance which is as high as four times per volume that of conventional lithium ion cells.
However, aluminium cells which have been experimentally made up to now are those cells which make use of liquid electrolytes such as non-aqueous electrolytes, ionic liquids (fused salts at normal temperatures) and the like. Aluminium cells in which solid electrolytes are used have never been in practical use.
Certain types of pyridinium or imidazolinium quaternary ammonium salts and aluminium chloride can form molten salts, which exhibit very high ionic conductivity. Thus, attention has been directed to these salts for use as a cell material. More particularly, if the molten salt is applied to a polymer solid electrolyte, it will be realized to provide polymer solid electrolytes which are in a solid polymer state and have such properties as of ionic liquid inherent to the molten salt, thereby obtaining a semi-solid, highly conductive polymer solid electrolyte.
However, the polymer solid electrolyte comprising the pyridinium or imidazolinium quaternary ammonium salt has the problem that it has the possibility of electronic conduction through .pi. electrons. For the application of this type of polymer solid electrolyte to electrochemical devices, mechanical strength and flexibility should be further improved.